1. Field of the Invention
The invention relates to the field of telecommunications technology, and specifically to digital telephony.
The invention can be used as an automatic branch exchange (PABX) in digital telephone networks.
2. Description of the Related Art
The new generation of switching facilities based on the use of digital transmission and switching methods is designed to improve the flexibility and cost-efficiency of telecommunication systems, reduce maintenance costs and labor comsumption, simplify manufacture, and enlarge the scope of services provided to subscribers. Currently, known in the art are a series of program-controlled electronic exchanges for switching digitized telephone signals (Siemens, Digital Electronic Switching System EWSD, 1987; GB, B, 2.021.901; U.S. Pat. No. 4,201,889).
These electronic automatic branch exchanges (EPABX) comprise a through-switching module with a group switching stage and units for interchange of telephone information and handshaking signals with other PABXs in the telephone network, and several subscriber concentration modules comprising analogue-to-digital and digital-to-analogue converters, a subscriber switching stage for digitized telephone signal switching, subscriber telephone sets for voice communication with other telephone sets and for handshaking via physical individual telephone lines, and transmit/receive units for interchange of information and signaling data between through-switching modules and subscriber concentration modules. The EPABX can be controlled from a host computer, but a more common practice is to provide control at the level of through-switching and subscriber concentration modules.
To reduce the overall length of cables to individual subscribers in some exchanges the entire subscriber concentration module or part of its equipment is remote to the EPABX and designed as remote digital concentrators linked to the EPABX via digital group communication lines. Some PABXs are equipped with special devices linking the subscriber's terminal to an individual digital communication line and thus widening the functional capabilities of this subscriber.
The common drawback of these known in the art digital PABXs is the relatively high cost of equipment, which is considerably higher than that of cross-bar systems. The inherent high cost of remote digital concentrators restricts the possible savings in cabling. Facilities linking digital data streams to a subscriber terminal are of sophisticated design due to the high transmission rate (192 Kbits per second), complexity of generating and separating digital data streams and difficulty of two-way transmitting them via pair telecommunications lines, resulring in their being affordable to a limited number of telephone networks subscribers.
In the description of this invention the term "digital telephone system" is used to designate a system comprising digital switching equipment and a number of telephone sets linked to it.
Known in the art is a digital telephone system (Telenokia, The DX=200 Electrontic ATX, 1985, Finland), wherein several Dx210 EPABXs are used as subscriber concentration modules linked to a DX=200 through-switching exchange used as the through-switching module.
This known in the art digital telephone system comprises a through-switching module with a group switching stage, connected to groups of terminal exchange sets designed to interface to subscriber concentration modules of this system and to other PABXs, a common signaling channel unit, line and multifrequency signaling units being connected to the group switching level. The through-switching module also comprises a control device (marker) connected via a control signal exchange bus to the group switching stage and to the common signaling channel and line signaling units. This known in the art digital switching system also comprises several subscriber concentration modules designed to concentrate subscriber channels, with each such module linked via its group input/outputs and intermediate group communications lines to the inputs/outputs of the through-switching module, namely to one of the groups of terminal exchange sets of the through-switching module. Each subscriber concentration module includes a subscriber switching stage connected by its inputs/outputs to groups of terminal exchange sets, to a group of subscriber modules designed to interface to telephone sets, to a handshaking signal generator, and to a common signaling channel unit. Each subscriber concentration module also comprises a control device (call processing unit) connected to the subscriber switching level and to the common signaling channel unit.
This known in the art digital telephone system also comprises remote subscriber modules with group subscriber inputs/outputs connected to one terminal exchange set of the subscriber concentration module. Each such remote subscriber module comprises a regenerator, a cycle synchronization signal receiver, and a pulse sequence generator, and also a cycle generator, group analogue-to-digital converters (ADC's) and digital-to-analogue converters (DACs), a concentrator multiplexing sixty four subscriber channels into a stream of samples from thirty channels, a switch routing the samples of thirty channels to sixty four subscriber channels, a processor, and sixty four subscriber sets each comprising a switching set, a differential balanced-loop system, a supply bridge, a signal amplifier, and a switching set control circuitry with appropriate links.
The remote subscriber module also comprises a call signal generator, a test signal generator, and a local power supply connected via the supply bridge to the differential system of each subscriber set. The subscriber module of the subscriber concentration module and the remote subscriber module provide communications between the subscriber concentration module and the telephone sets, these latter being of any type allowing transmission and reception of analogue signals and comprising a cradle switch, electroacoustic call signal transducer, earphone, microphone, dialing assembly, and electronic key connected to the input/output of the telephone set via a voltage regulator.
Outgoing communications are established by lifting the handset from the cradle, thus setting the cradle switch to its enabled state and causing current to flow from the remote subscriber module via a subscriber line. This current is detected by the switching set control circuitry, which generates an output signal indicating subscriber activity which is passed via a separate channel by the call processor to the call processor unit in the subscriber concentration module via its subscriber switching stage, this latter commanding the remote subscriber module processor to enable the readiness signal, indicating that the exchange is ready to accept the call. During dialing the electronic key breaks the supply circuit, thus causing generation of packets of currentless pulses, whose number in each packet corresponds to the digit being dialed.
The digits dialed are received by the call processor unit of the subscriber concentration module on arrival of a signal from the remote subscriber module processor.
On reception of the complete number being called, the call processor unit of the subscriber concentration module passes a signal to the control device (marker) in the through-switching module via the common signaling channel unit, the subscriber switching stage and terminal exchange sets of the subscriber concentration module, the terminal exchange sets of the through-switching module and the latter's group switching stage and common signaling channel unit. The through-switching module's marker controls handshaking between telephone exchanges and on completion of the handshaking procedure this marker and the subscriber contration module's call processor unit establish a connection between subscribers in the appropriate subscriber concentration module and through-switching module.
The voice signal from the microphone of the telephone set is passed in analogue form via the exchange set of the remote subscriber module and the subscriber line to the analogue-to-digital converter in the remote subscriber module and is included in digital form into the group signal transmitted via the group line and terminal exchange sets to the subscriber switching level in the subscriber concentration module, wherein the traffic is concentrated and the group signal is passed to the terminal exchange sets of the through-switching module.
In a similar way, the signal from the microphone of the called subscriber is passed via the through-switching module and digital intermediate group line to the subscriber concentration module, then to the remote subscriber module, wherein it is converted into an analogue signal in the digital-to-analogue converter and passed to an appropriate telephone set via the subscriber's set.
However, operation of this known in the art system is hampered by a number of factors.
First, due to the considerable spread in subscriber line lengths and spread in resistances the differential system of each remote subscriber module has to be balanced. The spread in subscriber line lengths also results in a spread of signal attenuation factors, thus necessitating a corresponding adjustment of the transfer factors in subscriber sets of remote subscriber modules. Both factors complicate the design of subscriber sets in remote subscriber modules.
Second place, in this system subscriber lines carry dialing and subscriber call signals of high amplitudes and therefore have to use switching sets with electromechanical and electronic keys. At the same time, speech signals are of considerably lower voltage amplitudes, so that the switching sets have to meet contradictory voltage specifications.
Third, remote subscriber modules perform digital-to-analogue signal conversion and vice versa for transmission via group subscriber lines, thus excluding the same group subscriber line being used to transmit the digital data stream directly to the telephone set and therefore complicating the use of digital subscriber equipment. This essentially restricts the functional capabilities of the digital telephone system.
Finally, considerable power consumption by the group analogue-to-digital and digital-to-analogue converters, and also in processors, analogue sets and traditional telephone sets, necessitates the use of local power supplies, thus resulting in greater dimensions of remote subscriber modules and requiring special space for its installation and thus hampering its use as a means of saving cabling.
The present invention is to provide a digital telephone system with the design configuration of remote subscriber modules allowing reception and transmission of speech signals, dialing signals, and call signals of a standard form, transmitted as a digital data stream, and with telephone exchange equipment of simpler design, to provide a wider scope of functional capabilities of the digital telephone system by connecting other types of subscriber digital equipment, and also to reduce the dimensions of the exchange equipment, the space required for its installation, and the length of communication lines, at the same time providing remote power supply to telephone sets and remote subscriber modules.